CompUSA was selling 512MB sticks of DDR2100/2700 PNY memory for $94.99 plus a $30 mail-in rebate. I bought it for my PB and my GFs iBook and we have noticed a performance boost. I don't know why you would drop performance points for more memory, though. I'm not really into the whole xbench performance thing.
If you don't experience crashes or kernal panics, then the memory is fine and I guess that is just how it goes (with your xbench score). But $100-150 is pretty average for 512MB sticks of notebook ram from what I have seen.